This invention relates to determining the gas or vapor, which may be referred to herein collectively as xe2x80x9cgasxe2x80x9d, transmission properties of packages and materials (for example, films or formed sheets) for use in packages, which may be referred to herein collectively as xe2x80x9cpackagingxe2x80x9d. More particularly, this invention relates to determining packaging transmission properties for one gas based upon packaging transmission data for another gas. Still more particularly, this invention relates to, but is not limited to, systems for determining the water vapor, oxygen, and carbon dioxide transmission properties of packaging based upon helium transmission data for such packaging.
Many products manufactured today, such as pharmaceuticals, medical devices and food products, are distributed in sealed packages that are intended to avoid adverse effects on the packaged products that may be caused by exposure to the air and moisture, such as oxidation or hydrolysis of the products, as well as the potential loss (or gain) of volatile components, usually of an organic nature. The packaging process for such products is generally conducted in a controlled atmosphere, such as dry air or an inert atmosphere, intended to preserve the product during storage. Thus, the filled and sealed package contains the product surrounded by the desired atmosphere. Examples of such sealed packages include xe2x80x9cblister packsxe2x80x9d and envelopes formed from plastic or metal films or multi-layered foils.
Package integrity is often important enough to the safety or utility of the packaged product to warrant testing and measurement, which may be performed during package design and development, packaged product production, and/or packaged product distribution and storage. Desirably, package testing can detect any sealing or other package problem so that it can be remedied quickly, before too many defective packages are produced, thus saving time and money and avoiding distribution of products with substandard packaging.
Package integrity is compromised by gas, or vapor, transmission through the package, which may occur in several ways. Gas transmission may occur by the process of leakage, in which gas travels through passages between interior and the exterior of the package that are large compared to the size of the gas molecules. Because packaging materials tend to be relatively free of holes that are large enough to permit leakage across the material, leakage occurs primarily through the seal areas between the components of the package. Gas transmission may also occur by the process of permeation, in which gas passes through the packaging materials by absorption at one surface of the material, diffusion through the material, and desorption at the other surface of the material. The total gas transmission of a package includes its leakage transmission and its permeation transmission.
Gases which often are of interest in packaging include oxygen, water vapor, and carbon dioxide, as well as other gases or vapors. These gases can have adverse effects on a packaged product and so may be excluded from the controlled atmosphere present when packages are sealed; they are also atmospheric constituents that can enter a sealed package from the surrounding air. Protection of the product from loss of volatile components is also sometimes required of a package. Various methods have been developed to measure the transmission rate of gases and vapors through finished packages or through the materials from which packages are made. One method is gravimetric, in which weight changes due to gas transmission through packaging are measured. For instance, if water vapor transmission rate through a film intended to be used in packaging as a moisture barrier is to be measured, a desiccant can be placed in a cell made of metal or another impervious material having an opening covered by the film to be tested. The cell is weighed and then placed in a high humidity atmosphere. Water vapor flow through the film is absorbed by the desiccant, which maintains a low partial pressure of water vapor inside the cell. Alternatively, water can be in the cell (test fixture) and the test fixture/s stored in a desiccator. The change in weight of the cell over time reflects the total amount of water vapor that has permeated through the film during that time, which when divided by the time yields the water vapor transmission rate. Another method is to place a source of a gas of interest on one side of a sample of a film to be tested, and convey the gas that permeates through the film to a detector that provides quantitative measurements of the gas of interest. Conveyance of permeated gas may be accomplished by passing a carrier gas over the film so as to pick up any gas of interest that has permeated through the film and carry it to the detector; such a carrier gas is selected so that the detector is not responsive to it or so that its effects on the detector can be distinguished from those of the gas of interest. Conveyance of permeated gas may also be accomplished by a vacuum pump disposed to draw permeated gas to the detector. Examples of detectors that have been used in this method include mass spectrometers, gas chromatographs, and infrared detectors.
The above-described prior art methods of determining gas transmission properties of packaging have various drawbacks. They are primarily suited to determining gas transmission properties of packaging materials, rather than packages, but it is often the performance of completed packages that is primarily of interest. They are often slow; it may take days, weeks, or months for the packaging materials to transmit enough gas of interest to obtain a significant response from the detector. Their sensitivity may be low, so that they may be unable to measure transmission rates for packaging materials with low gas transmission rates. It is expensive to carry out the measurement methods of the prior art; not only is expensive equipment required, but different equipment is required for each gas of interest, and low throughput makes the cost per measurement high. It is therefore a general object of the present invention to provide methods and apparatus for easily, quickly, accurately and relatively inexpensively determining packaging leakage and permeation of gases of interest, including, but not limited to, oxygen, water vapor, and carbon dioxide.
In accordance with one aspect of the present invention, data is obtained as to the total transmission rate of a test gas through a package. The test gas permeation rate of the package is determined based upon test gas permeation data for the materials from which the package is made and the geometry of the package. The test gas leakage rate of the package is determined by subtracting the determined test gas permeation rate from the test gas total transmission rate. Having determined the leakage and permeation components of the total gas transmission rate for the test gas, the leakage rate for a gas of interest can be determined based upon the leakage rate for the test gas, the permeation rate for the gas of interest can be determined based upon the permeation rate for the test gas, and the total transmission rate for the gas of interest can be determined by adding the determined leakage and permeation rates for the gas of interest. Helium is preferred as a test gas, i.e. helium transmission data is preferred as test gas transmission data. The invention may be implemented using a computer operating on test gas and gas-of-interest leakage and/or permeation data for the materials from which a package is constructed, package structure data, and test gas total transmission rate data to determine the package gas transmission properties for the gas of interest. By obtaining data enabling correlation of leakage and permeation properties of test and other gases of interest, data from a single measurement of a package""s total test gas transmission rate enables determination of the leakage and permeation rates of many gases of interest. Similarly, multi-gas permeation data can be obtained for permeable films and formed sheets. Further in accordance with the invention, the transmission rates determined for the gas or gases of interest may be used with other package-related data to determine properties such as shelf life of a packaged product or sensitivity of a product to gases of interest.
The oxygen, water vapor, carbon dioxide, and other gas-of-interest transmission properties of a packaging material such as a formed sheet are also often of interest. Therefore, in accordance with another aspect of the invention, data is obtained as to the transmission rate of a test gas through a specimen of the packaging material, and the specimen""s permeation of the gas of interest is determined based on its permeation of the test gas, in accordance with data correlating the permeation of the gas of interest and permeation of the test gas for the packaging material.